These tools let developers write their app once to make the app compatible with all the major versions of Windows, Windows Phone, Xbox.

But that's not the same as consolidating the operating system itself, which is what Nadella just promised to do.

"This means one operating system that covers all screen sizes," Nadella said to analysts on the quarterly conference call. "We will streamline the next version of Windows from three operating systems into one single converged operating system for screens of all sizes."

He explained, "In the past we had multiple teams working on different versions of Windows. Now we have one team with a common architecture. This allows us to scale, create Universal Windows Apps."

This is a major departure from Apple's style and from Microsoft's previous approach.

Under previous CEO Steve Ballmer, Microsoft took a similar approach of multiple operating systems that couldn't always run each other's apps.

To be clear, Microsoft will still sell different editions of its operating systems, for instance, a Windows Pro edition, a Windows Enterprise edition, a cheap one tightly bound with Bing for low-price devices (which we'll see by the holiday season, Nadella says). But under the hood, they will be more alike than different.

And the company will "unify our stores, our commerce and developer platforms," Nadella promised.

This is hugely important to developers. A good billion people use Windows every day. But as long as Windows was chopped up into a bunch of different operating systems that didn't work together, all of those PC customers didn't add up to phone, tablet, or living room console app customers.